1. Field of the Invention
The present invention generally relates to a laminated diffractive optical element in which a low-refractive high-dispersive material and a high-refractive low-dispersive material are laminated substantially without voids, and a resin composition therefor.
2. Description of the Related Art
In conventional refractive optical systems using light refraction, chromatic aberration may be reduced by combining lenses composed of glass materials with different dispersion characteristics. For example, in an objective lens of a telescope or the like, a glass material with low dispersion may be used for a positive lens, a glass material with high dispersion may used for a negative lens, and a chromatic aberration appearing on the axis can be corrected by using the combination of such lenses. However, when the configuration or number of lenses is limited, or when a limitation is placed on the glass materials that can be used, chromatic aberration is sometimes difficult to correct completely.
A. D. Kathman and S. K. Pitalo, “Binary Optics in Lens Design”, International Lens Design Conference, 1990, SPIE Vol. 1354, p. 297-309 discloses a method for inhibiting chromatic aberration with several small lenses by using a combination of a refractive optical element having a refractive surface and a diffractive optical element having a diffraction grating.
This method uses a physical phenomenon where the directions in which color aberration occurs in a light beam of a certain reference wavelength at a refractive surface and a diffractive surface of the optical elements are opposite to each other. Further, a characteristic similar to that of an aspherical lens can be demonstrated by changing the period of a diffraction grating formed continuously with a diffractive optical element.
However, one light beam falling on a diffractive optical element is divided by the diffraction action into a plurality of light beams with different orders of diffraction. In this case, the diffracted light having an order of diffraction different from that of the designed order of diffraction may form an image in a location other than that in which the image is formed by the light beam with the designed order of diffraction, thereby causing flare.
U.S. Pat. Nos. 5,847,877 and 6,262,846 indicate that high diffractive effect can be realized within a wide wavelength range by optimizing the refractive index dispersion of optical elements and the shape of a grating formed on the boundary surface of optical elements. By concentrating the light flux of a useful wavelength range at a specific order (referred to hereinbelow as “designed order”) of diffraction, the intensity of diffracted light with other orders of diffraction is reduced, and flare generation may be inhibited.
More specifically, the configuration described in U.S. Pat. No. 5,847,877 uses BMS81 (nd=1.64, νd=60.1; manufactured by OHARA Company) and a plastic optical material PC (nd=1.58, νd=30.5; manufactured by Teijin Chemical Ltd.), and that described in U.S. Pat. No. 6,262,846 uses COO1 (nd=1.5250, νd=50.8; manufactured by Dainippon Inks And Chemicals Co., Ltd.), a plastic optical material PC (nd=1.58, νd=30.5; manufactured by Teijin Chemical Ltd.), and BMS81 (nd=1.64, νd=60.1; manufactured by OHARA Company).
The Abbe number (νd) can be calculated with the following Equation (1).νd=(nd−1)/(nf−nc)  (1)
nd: refractive index of d line (587.6 nm).
nf: refractive index of f line (486.1 nm).
nc: refractive index of c line (656.3 nm).
With the object of improving optical properties of the above-described diffractive optical element, the inventors have studied optical materials that have been marketed, researched, or developed. The distribution shown in FIG. 1A represents the results obtained. The materials of the laminated diffractive optical elements described in U.S. Pat. Nos. 5,847,877 and 6,262,846 are also included in the distribution shown in FIG. 1A.
U.S. Pat. No. 5,847,877 also discloses using a combination of a diffractive optical element molded from a material with a relatively low dispersion of refractive index, and a diffractive optical element molded from a material with a high dispersion of refractive index, in order to obtain a configuration having high diffraction efficiency within a wide wavelength range.
Thus, where the difference in dispersion of refractive index between a material with a high dispersion of refractive index and a material with a low dispersion of refractive index is large, the diffraction efficiency of the optical element configured by the materials is high and the angle of field is increased. Therefore, in order to correct chromic aberration with high accuracy, it may be necessary to use a material with a higher dispersion of refractive index (small Abbe number) and a material with a lower dispersion of refractive index (large Abbe number).
U.S. Pat. No. 6,059,411 discloses an optical material with the following relationship between refractive index (nd) and Abbe number (νd): nd>−6.667×10−3 νd+1.70 and the following relationship between the second-order dispersion (θg, F) and Abbe number (νd): θg, F≦−2νd+10−3+0.59. By satisfying the conditions represented by these formulas, it may be possible to increase the diffraction efficiency in the entire visible region.
Further, the optical material in U.S. Pat. No. 6,059,411 is a composite material in which a transparent electrically conductive metal oxide having high dispersion of refractive index and a low second-order dispersion characteristic is mixed and dispersed in the form of fine particles in a binder resin. ITO, ATO, SnO2, ZnO, and the like are disclosed as transparent electrically conductive metal oxides.
A very strong demand has been created in recent years for miniaturization of components in optical devices using optical elements. Accordingly, developments aimed at significant reduction in thickness of optical elements have been advanced. Thus, a multilayer diffractive optical element of a voidless type, which is not the above-described laminated diffractive optical element in which a void is present between a first-layer diffractive optical element and a second-layer diffractive optical element, has been developed.
In the case of a multilayer diffractive optical element of a voidless type, in addition to a dispersion characteristic of refractive index, a refractive index characteristic of two diffractive optical elements is also important from the standpoint of increasing the diffraction efficiency. Thus, the higher the difference in refractive index between the two diffractive optical elements, the higher the diffraction efficiency. Furthermore, as in the configuration described in U.S. Pat. No. 6,059,411, it may be preferred that the second-order dispersion characteristic (θg, F) representing a linearity of the dispersion of refractive index be as small as possible.
However, in the case of optical materials described in U.S. Pat. Nos. 5,847,877 and 6,262,846, the lowest refractive index is about 1.52.